AutoClearedValue: A lazy property that gets cleaned up when the fragment's view is destroyed.

AutoClearedValue.kt

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.example.github.util

import androidx.fragment.app.Fragment
import androidx.lifecycle.DefaultLifecycleObserver
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.observe
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty

/**
 * A lazy property that gets cleaned up when the fragment's view is destroyed.
 *
 * Accessing this variable while the fragment's view is destroyed will throw NPE.
 */
class AutoClearedValue<T : Any>(val fragment: Fragment) : ReadWriteProperty<Fragment, T> {
    private var _value: T? = null

    init {
        fragment.lifecycle.addObserver(object: DefaultLifecycleObserver {
            override fun onCreate(owner: LifecycleOwner) {
                fragment.viewLifecycleOwnerLiveData.observe(fragment) { viewLifecycleOwner ->
                    viewLifecycleOwner?.lifecycle?.addObserver(object: DefaultLifecycleObserver {
                        override fun onDestroy(owner: LifecycleOwner) {
                            _value = null
                        }
                    })
                }
            }
        })
    }

    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        return _value ?: throw IllegalStateException(
            "should never call auto-cleared-value get when it might not be available"
        )
    }

    override fun setValue(thisRef: Fragment, property: KProperty<*>, value: T) {
        _value = value
    }
}

/**
 * Creates an [AutoClearedValue] associated with this fragment.
 */
fun <T : Any> Fragment.autoCleared() = AutoClearedValue<T>(this)

In a fragment you can do the following:

class RepoFragment : Fragment() {
    ...

    var binding by autoCleared<RepoFragmentBinding>()

    private var adapter by autoCleared<ContributorAdapter>()

    ...
}

This is taken from GithubBrowserSample by Google